This invention relates to hydraulic power units of the type that incorporate a cylinder block containing reciprocating pistons that are in operational engagement with a swashplate through pivotally attached slippers. Such hydraulic power units may operate as either pumps or motors. Specifically, this invention is concerned with an improved piston for use in such hydraulic power units.
Typically, the pistons that are utilized in hydraulic power units of the aforementioned type have been manufactured from a composition of metallic components, usually steel, secured together to form a solid piston. Such pistons typically have a closed or solid end opposite the end that engages the slipper and the swashplate. Closed end pistons are shown in U.S. Pat. No 3,319,575 and in U.S. Pat. No. 6,293,185. Such pistons typically have a fluid conduit through a center bore that traverses the central axis of the piston. Fluid conduits extending the length of the piston are shown in U.S. Pat. No 3,319,575, U.S. Pat. No. 3,882,762, and in U.S. Pat. No. 6,293,185.
It is desirable in a hydrostatic motor or pump to reduce the amount of xe2x80x9cdead volumexe2x80x9d in the piston bore. xe2x80x9cDead volumexe2x80x9d is the volume of oil in a piston bore that converts mechanical energy to fluid energy. The amount of energy loss can become very high at high pressures. xe2x80x9cDead volumexe2x80x9d also has a negative effect on the variability of swashplate forces that are transferred to the swashplate from the piston/slipper assembly. The lower the amount of xe2x80x9cdead volumexe2x80x9d the lower the variability of the swashplate forces. It is desirable to reduce the swashplate force variability in order to optimize the controllability of the entire hydrostatic unit. Reducing the xe2x80x9cdead volumexe2x80x9d can be achieved in other ways, but these are generally costly alternatives. It is desirable to reduce the dead volume for the least amount of cost.
It is preferable for the interior of a piston to have an open cavity and a fluid conduit. Such cavity and fluid conduits have typically been created by the geometry of the component parts that are joined to form the piston. It is advantageous for the hollow cavity and fluid conduit to be formed in a single process and it is further advantageous for the piston to be of a solid unitary construction.
Therefore, it is a principal objective of this invention to provide a piston with a stem that reduces dead volume in the bore.
A further object of this invention is to provide a piston with a stem that reduces the dead volume in the bore that is economical of manufacture.
A piston for a cylinder block of a hydraulic power unit has a cylinder body with an open end and a closed end terminating in a surface configured to receive a slipper to engage a swashplate. The piston body is in the form of a hollow cylinder with a hollow stem located at its core. The connection between the piston and the slipper to the swashplate may take the form of either an external ball on the piston to be received by a corresponding socket on the slipper, or a socket on the piston to receive a corresponding ball on the slipper. The piston is held against the slipper to the swashplate by force from an internally mounted spring. The piston incorporates a fluid conduit through a stem in the piston, which provides a path for pressurized fluid to flow. The stem effectively fills much of the dead space within the piston.
The piston is created of a unitary construction, preferably from casting.